1. Field of the Invention
The present invention relates to the manufacture of an extremely thin, high flute density, corrugated paperboard. In particular, the invention disclosed herein relates to a corrugated paperboard suitable for use on high resolution offset lithographic printing operations, and to a method of manufacturing such a product.
2. Background of the Related Art
Corrugated paperboard, commonly referred to as cardboard, is in its most basic form structurally comprised of three layers of sheet paper stock in which a middle corrugated layer of paper is sandwiched between two flat paper liners. The middle layer of paper is corrugated by forming in it a regular pattern of alternating ridges and grooves. The ridges and grooves, commonly referred to in the industry as flutes, are then glued to the interior surfaces of the two flat paper liners. In comparison to solid board of the same thickness, a corrugated paperboard uses much less material and therefore is typically much less costly, and corrugated paperboard is usually much stronger structurally.
Corrugated paperboard has long been known and has many uses. It is widely used in making containers, i.e., boxes, for packaging and shipping a wide variety of goods. However, corrugated paperboard also has its limitations. For instance, corrugated paperboard is usually not well suited for use in high quality graphics printing operations. With respect to the outer surface of the liner on a corrugated paperboard, the area between adjacent flutes is unsupported and therefore too flimsy to print on. It is simply impossible to precisely align the different colored dots of ink on a multi-color press. The ink dots run together, resulting in a blurred image. Current grades of corrugated paperboard are also too thick to run on standard offset printing presses. Most printing presses require a printing stock having an approximate caliper thickness of 0.040 inches or less. One of the thinnest known grades of corrugated paperboard is a light weight laminated paperboard produced by Book Cover Inc. (see U.S. Pat. No. 4,931,346). That product in its thinnest grade is 0.060 inches, or 1.5 mm, which is 50% too thick for use as printing stock in offset presses.
Corrugated paperboard is ordinarily produced on large paper web processing machines on which large rolls of raw paper stock are unwound and processed continuously to produce the product. Depending on the grade of product being made and the efficiency of the operation, corrugated paperboard is preferably produced at speeds ranging between 300 and 800 linear feet per minute. However, conventional corrugating operations are simply incapable of producing a corrugated paperboard suitable for use in offset lithographic printing operations. There are many reasons for this, most of which may be generally categorized as either a physical limitation of the paper stock used in corrugating operations, or in a limitation in the process of gluing the multiple layers of paper together to form the corrugated paperboard.
To manufacture a very thin corrugated paperboard, a very thin, flexible paper stock must be used, particularly for the middle layer. However, paper stock which is too thin and too flimsy will become wrinkled and ripped on most conventional corrugators. The adhesive compounds used on most conventional corrugating operations typically use a low solid content and high moisture content formulation. In the manufacture of corrugated paperboard, the product is subjected to a high degree of heat and pressure in order to evaporate the moisture and thus dry the glue. In larger grades of corrugated paperboard, the moisture is exhausted through the flutes out through the side edges of the product. To manufacture a corrugated paperboard with an approximate caliper thickness of less than 0.040 inches, which would be required to make it usable in offset printing presses, the flutes are simply too small to function as exhaust channels for evaporating the moisture in the glue. As a result, the moisture in the adhesive formulation flashes thereby causing the bonds between the various layers of paper to break apart and separate. Because of these and other process limitations, conventional corrugating operations are simply incapable of producing grades of corrugated paperboard suitable for use in offset printing operations.